The present invention is useful in apparatus for continuously fabricating photovoltaic devices on a web of magnetic substrate material by depositing thereonto successive amorphous-silicon alloy semiconductor layers in each of at least two adjacent, interconnected deposition chambers. The composition of each layer is dependent upon the particular gases introduced into and isolated from each of the deposition chambers. More particularly, the deposition chambers are operatively interconnected by a gas gate which includes a relatively narrow passageway (1) through which the web of substrate material passes; and (2) adapted to substantially isolate the processes gases introduced into the first chamber from the process gases introduced into the adjacent deposition chamber. As disclosed in U.S. Pat. No. 4,462,332 to Nath et al., assigned to the assignee of the instant application and the disclosure of which is hereby incorporated by reference, it has been determined that despite the relatively small size of the gas gate passageway, dopant process gases introduced into one deposition chamber back diffuse into the adjacent chamber, thereby contaminating the process gases introduced thereinto and the semiconductor layer deposited in said adjacent chamber. The '332 patent disclosed apparatus (namely ceramic magnets positioned above the gas gate passageway for urging the magnetic substrate upwardly) by which the height dimension of the passageway could be reduced. The reduction in the height dimension of the passageway provided for a corresponding reduction of the back diffusion of dopant gases for a given flow rate, thereby decreasing the contamination of the process gases introduced into the intrinsic deposition chamber.
However, because the magnets urge the substrate into sliding contact with the upper passageway wall, frictional abrasion between the wall and the unlayered side of the substrate causes problems with the deposition apparatus such as, for example, wear of the upper passageway wall of the gas gate. Also, abraded particles of substrate and passageway wall material collect in the passageway and deposition chambers causing scratching of the layered side of the substrate and co-depositing with the semiconductor material, which causes short circuiting due to the protruding particles which cannot be fully covered by a one micron thick semiconductor alloy layer. The abrasion, in addition to being detrimental to the semiconductor layer and the equipment, limits the minimum thickness of the web of substrate material which can be realistically used due to possible tearing.
Additionally, as was disclosed in U.S. Pat. Nos. 4,438,724 and 4,450,786 each to Doehler et al., both assigned to the assignee of the instant application and the disclosures of which are hereby incorporated by reference, when the web of substrate material is urged against the upper wall of the passageway, the passageway is divided by the web of substrate into a relatively narrow upper portion, between the substrate and the upper passageway wall, and a relatively wide lower portion, between the substrate and the lower passageway wall. Also, irregular spacing between the substrate and the upper passageway wall occurred because waffling (warping) of the web of substrate material could not be entirely eliminated by the attractive force of the magnets. Much of the warping of the substrate is caused by heating of the substrate in the deposition chambers. The process gases, being inherently viscous (and especially viscous at the elevated deposition temperatures employed with glow discharge deposition processes), are unable to travel through the narrow upper portion with sufficient velocity to prevent cross-contamination of process gases from one deposition chamber to the other. It was to the end of decreasing the amount of cross-contamination of process gases through the narrow upper portion between the unlayered side of the substrate and the upper passage wall that the '724 and '786 patents were directed.
In the past, considerable efforts have been made to develop processes for depositing layers of amorphous semiconductor alloy material, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the fabrication of p-i-n-type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. For many years such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films using silane (SiH.sub.4) gas and hydrogen gas as precursors. The material so deposited is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3) for n-type or diborane gas (B.sub.2 H.sub.6) for p-type conduction, is premixed with the silane gas. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type.
It is now possible to prepare greatly improved amorphous silicon alloy materials, that have significantly reduced concentrations of localized states in the energy gap thereof, while providing high quality electronic properties by glow discharge as is fully described in U.S. Pat. No. 4,226,898 to Ovshinsky et al., and by vapor deposition as described in U.S. Pat. No. 4,217,374 to Ovshinsky et al., both assigned to the assignee of the instant application and the disclosures of which are hereby incorporated by reference. As disclosed in these patents. fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of localized states therein and facilitates the addition of other alloying materials, such as germanium. Activated fluorine readily diffuses into, and bonds to, amorphous silicon in a matrix body to substantially decrease the density of localized states therein. This is because the small size of the fluorine atoms enable them to be readily introduced into an amorphous silicon matrix. The fluorine is believed to bond to the dangling bonds of the silicon and form a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (V.sub.oc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell. the overall open circuit voltage is the sum of the open circuit voltages of each cell while the short circuit current remains substantially constant.
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells may now incorporate fluorine to reduce the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, the cell material maintains high electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponse application.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in U.S. Pat. Nos. 4,440,409; 4,542,711; 4,410,558; 4,438,723; and 4,492,181 each of which is assigned to the assignee of the instant application and the disclosures of which are hereby incorporated by reference. As disclosed in these patents, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor layer. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon semiconductor alloy material, the second chamber is dedicated for depositing an intrinsic amorphous silicon semiconductor alloy material, and the third chamber is dedicated for depositing an n-type amorphous silicon semiconductor alloy material. Since each deposited semiconductor alloy material, and especially the intrinsic semiconductor alloy material must be of high purity, the deposition environment in the deposition chamber is isolated from the doping constituents within the other chambers to prevent cross-contamination of doping constituents into the intrinsic process gases in the intrinsic chamber. In the previously mentioned patents, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which (1) a unidirectional flow of process gases between deposition chambers is established, and (2) an inert gas may be "swept" along the web of substrate material. The gas gate disclosed in previously mentioned U.S. Pat. No. 4,462,332 contemplated the creation of a plurality of magnetic fields adapted to urge the magnetic web of substrate material against a wall of the gas gate passageway opening so that the height dimension of the passageway opening could be reduced. The reduced height of the opening correspondingly decreased the quantity of process gas, which would otherwise diffuse from the dopant deposition chambers to the intrinsic deposition chamber, without correspondingly increasing the risk that the amorphous semiconductor layers deposited on the substrate would contact and be damaged by a wall of the gas gate passageway opening.
While the magnetic gas gate disclosed in U.S. Pat. No. 4,462,332 reduced the height dimension of the passageway opening, and thereby correspondingly reducing the diffusion of dopant gas, this gas gate design caused two additional problems, (1) the aforementioned problems of frictional, and (2) it divided the passageway into wide and narrow portions, as discussed hereinabove. Regarding the latter of these problems, the velocity of the inert sweep gas and residual process gases traveling through the wide lower portion is sufficiently great to substantially prevent cross-contamination of dopant gases into the intrinsic chamber. However, due to the viscosity of the process gases, the drag on the sweep gases along (1) the upper passageway wall and (2) the unlayered surface of the substrate (which define the relatively narrow upper portion of the passageway) results in a relatively low velocity flow therethrough. Accordingly, an undesirably high amount of dopant process gas is able to diffuse into the intrinsic chamber through the narrow upper portion.
The problem of cross-contamination was reduced in U.S. Pat. Nos. 4,438,724 and 4,450,786 by providing a plurality of elongated grooves (extending the entire length of the gas gate passageway opening) from the dopant deposition chamber to the adjacent intrinsic deposition chamber in the wall of the passageway opening above the web of substrate material. In this manner, a plurality of spaced, relatively high velocity flow channels were provided in the space between the unlayered surface of the web of substrate material and the upper wall of the passageway opening. Because the channels were relatively high, the sweep gases and residual process gases flowed unidirectionally therethrough at substantial velocities despite the drag incurred as said gases contacted the passageway wall and the substrate surface. While the gas gate of the '724 and '786 patents reduced the problem of cross-contamination through the aforementioned narrow upper section, it failed to reduce the problem of frictional abrasion between the unlayered side of the substrate and the upper passageway wall.
The magnetic roller gas gate of our copending U.S. application Ser. No. 07/903,417, filed Jun. 24, 1992 and hereby incorporated by reference, substantially reduced the frictional abrasion between the unlayered side of the substrate and the passageway wall without substantially increasing in the cross-contamination of process gases between deposition chambers.
While the magnetic roller gas gate of the '417 application reduced the frictional abrasion problem and did not increase the cross contamination problem, the gas gates of the prior art cannot be used to operatively interconnect regions having a pressure differential between the chambers of greater than about 10%. However in many instances, it is desirable, if not essential, to interconnect two processing chambers having pressure differentials of greater than an order of magnitude (i.e., such as pressures of 10.sup.-1 and 10.sup.-3 Torr respectively). Therefore, it is an object of the instant invention to fabricate a gas gate which incorporates the reduced frictional abrasion of the substrate and low cross contamination of the magnetic roller gas gate of the '417 application and has the additional ability to be used to interconnect regions having pressure differentials greater than about one order of magnitude.
Although the foregoing discussion dealt with a single dopant deposition chamber and an adjacent intrinsic deposition chamber, it should be apparent that other deposition chambers may be operatively connected by the gas gates of the present invention. For example, a p-type deposition chamber may be connected on one side of the intrinsic deposition chamber and an n-type deposition chamber may be connected to the other side of the intrinsic deposition chamber so as to produce a p-i-n type semiconductor device. Alternatively, a plurality of these triads of deposition chambers could be interconnected to produce a plurality or p-i-n-type cells. Similarly, a chamber in which the transparent conductive oxide layer (discussed hereinafter) is added atop the uppermost amorphous semiconductor alloy layer may be operatively connected by the improved gas gates, disclosed herein, to the final deposition chamber. Since it would be undesirable to have gaseous constituents or other contaminants introduced into the transparent conductive oxide (TCO) deposition chamber (typically at a pressure of about 10.sup.-3 Torr pressure) cross-contaminated into the RF glow-discharge dopant deposition chamber (typically at a pressure of about 1 Torr pressure), the improved gas gate of the present invention can also be employed between the TCO chamber and the final dopant deposition chamber. For that matter, the improved gas gate of the instant invention is applicable between all chambers of the continuous production apparatus so as to produce high quality amorphous photovoltaic devices.